A review of helical milling process

Author’s Accepted Manuscript A REVIEW OF HELICAL MILLING PROCESS Robson Bruno Dutra Pereira, Lincoln Cardoso Brandão, Anderson Paulo de Paiva, João Roberto Ferreira, J. Paulo Davim www.elsevier.com/locate/ijmactool

PII: DOI: Reference:

S0890-6955(17)30070-6 http://dx.doi.org/10.1016/j.ijmachtools.2017.05.002 MTM3259

To appear in: International Journal of Machine Tools and Manufacture Received date: 6 January 2017 Revised date: 28 April 2017 Accepted date: 5 May 2017 Cite this article as: Robson Bruno Dutra Pereira, Lincoln Cardoso Brandão, Anderson Paulo de Paiva, João Roberto Ferreira and J. Paulo Davim, A REVIEW OF HELICAL MILLING PROCESS, International Journal of Machine Tools and Manufacture, http://dx.doi.org/10.1016/j.ijmachtools.2017.05.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A REVIEW OF HELICAL MILLING PROCESS Robson Bruno Dutra Pereira1*, Lincoln Cardoso Brandão1, Anderson Paulo de Paiva2, João Roberto Ferreira2, J. Paulo Davim3 1

Department of Mechanical Engineering, Federal University of São João Del-Rei, 170 Frei Orlando Square, São João del Rei, MG 36880-000, Brazil

2

Institute of Industrial Engineering and Management, Federal University of Itajubá, 1303 BPS Avenue, Itajubá, MG 37500-903, Brasil 3

Department of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal *

Corresponding author. Tel.: +55 32 99140 9788; fax: +55 32 3379 2525. [email protected]

Abstract Helical milling is an alternative hole-making machining process which presents several advantages when compared to conventional drilling. In the helical milling process, the tool proceeds a helical path while rotates around its own axis. Due to its flexible kinematics, low cutting forces, tool wear, and improved borehole quality may be achieved. This paper presents a review of the helical milling process. As a first paper aiming to describe the current state of the art of helical milling process, the recent works about this process were summarized to point out the future trends in this field. Initially, the advantages of the helical milling were presented with regard to conventional drilling. Subsequently, the kinematics of the process was presented to standardize the nomenclature and to provide knowledge about the movements and parameters of helical milling. It was demonstrated the feed velocity decomposition in frontal and peripheral directions. Undeformed chip and cutting volumes of frontal and peripheral cut were described, and the ratio between the cutting volumes removed by frontal and peripheral cut was demonstrated to be dependent only of the borehole and tool diameters. Cutting forces and temperature studies were also summarized, corroborating that the helical milling is a smooth holemaking process. Afterward, tool life and wear studies in helical milling were summarized, testifying that the tool wear evolution can be monitored in frontal and peripheral cutting edges, with frontal cutting edges, in most cases, defining the tool life. Some statistical and soft computing applications on helical milling were also mentioned. To provide initial guidelines for applying helical milling, a screening of the current literature was performed summarizing equipment and cooling techniques used, and the levels of cutting conditions of helical milling applied for hole-making different materials. The quality of boreholes obtained by helical milling was assessed in terms of dimensional, geometrical, and microgeometrical deviations, besides burr and delamination levels, assuring that it can be obtained finished boreholes with helical milling. In the conclusions, future possibilities on research about helical milling were pointed out. This general review of helical milling may be referenced as a summary of the current results obtained in experimental and theoretical studies and to provide future research needs and opportunities. Keyworks: Helical milling, orbital drilling, hole-making, borehole quality.

1. INTRODUCTION Helical milling is a hole-making process in which the milling tool proceeds a helical path while rotates around its own axis, presenting several advantages in relation to conventional drilling. The helical path can be decomposed into axial and tangential directions, combining frontal and peripheral cutting. Helical milling offers many different strategies that allow to create complex borehole geometries including the production of different borehole diameters, conical holes, complex tapered holes, perform finishing operations without changing the tool, due to the possibility of adjusting the eccentricity of the tool centre point to borehole centre [1]. Helical milling process has been applied for borehole generation in difficult-to-cut materials, such as Carbon Fiber reinforced Plastic (CFRP), Ti-alloys, hard materials, and other aerospace materials [2–8]. Throughout the helical milling hole-making process, satisfactory results have been obtained in terms of borehole dimension, geometry, and roughness [3,9,10]. The helical milling was labelled as a sustainable hole-making process [11]. Helical milling when performed with specific devices and industrial robots is generally referred as orbital drilling [12]. This paper describes the state of the art of helical milling process, covering a discussion about advantages and drawbacks of the process to justify its importance, besides elucidating recent applications and performance considering different outcomes. Initially, the advantages of the process are highlighted. Subsequently, helical milling kinematics is discussed, to describe the movements, process parameters, undeformed chip geometry, and cutting volumes. Cutting forces modelling and its effects in the process and in product quality are summarized. Temperature and its influence in process performance are addressed. Helical milling statistical and soft computing analysis, modelling, and optimisation studies are quoted to summarize different recent applied approaches. Tool life and wear are addressed to summarize the wear mechanisms and types, in different tool and workpiece materials and tool geometry, aiming to improve tool life. Borehole quality in terms of dimensional error, geometrical deviation, surface roughness, besides burr and delamination in CFRP are discussed to evaluate the possibilities of this hole-making process. Helical milling applications are also addressed, to resume the recent experimental applications of helical milling available on literature and allow the reader to know the effect of the process parameters in the outputs, allowing the experimenter to choose the levels of the cutting parameters for different materials and applications. Finally, conclusions are made, indicating future works possibilities, to give to the researchers the opportunity to explore new perspectives and deepen specific aspects of the helical milling process. 1.1. Helical milling advantages The drilling process is the most important machining process for obtaining boreholes [5,13–17]. According to Tonshoff et al. [18], drilling comprises 25 percent of cycle time and 33 percent of the total number of operations, Wang et al. [7] assures that hole-making is about 50 percent of total processing in machining scope, besides it is one of the last operations in the manufacturing of a part, demanding reliability due to the high added value related to previous machining operations. However, the drilling process presents several known drawbacks, such as high loads over the workpiece in axial direction, high thrust force, burr formation, work material breakout at the hole exit, deformation of the hole’s periphery, difficult chip evacuation, difficult heat dissipation, poor dimensional and

geometrical accuracy, poor surface quality, catastrophic tool wear [3,12,19,20]. Consequently, conventional drilling is associated with rework, low process capability, structural impairment and, consequently, extra costs [21]. These challenges, according to Iyer et al. [3], are related to the drilling removal process mechanics, in which the cutting speed approaches zero in the vicinity of the drill centre, and the material removal process in this region is realised by extrusion rather than cutting. Chip evacuation is also a limiting factor in conventional drilling, once the chips are transported laterally before being evacuated through the flutes. Conventional drilling is yet addressed as a traditional hole-making operation of CFRPs, Ti-6Al-4V and CFRP/Ti stacks [22–24]. However, in the case of composite materials, conventional drilling can lead to specific defects such as delamination, matrix burnout and fibber pullout [25]. Considering these problems, drilling is not a competitive operation in the aeronautical industry [26]. Besides, it is difficult to use standard industrial robots to automate drilling in the aerospace industry. The stiffness of the standard robotic device is not sufficient to resist the deflections caused by the cutting forces from the drilling process, making it difficult to achieve tight tolerances [27]. With consideration to the current trend of manufacture components after heat treatment, in hard machining the drawbacks of drilling process are exacerbated [3], once to drill materials with hardness approaching 60 HRC can be challenging [13]. Helical milling is an innovative machining process for hole-making which can be used as an alternative to conventional drilling [4,8,28–30]. Considering the helical feed in addition to the tool rotation, material removal near the borehole centre occurs by cutting rather than extrusion, differently of drilling, generating low thrust forces [3,31]. The process kinematics allows for obtaining different boreholes diameters without changing the tool, saving cycle time due tool changes [9,12,26,29,32], and avoiding additional finishing process of boreholes such as re-drilling, reaming and countersinking [19]. With a slight change in the helical diameter, there is the possibility of online correction of borehole dimensional and geometrical errors [33]. It is an important advantage of helical milling, once in drilling, as the drill life is approaching its end, borehole dimensional and geometrical deviations are attested, as confirmed by Coldwell et al. [13]. Considering the advantages related to the flexible kinematics of the helical milling, this process was referred as an eco-friendly hole-making process [11]. Orbital drilling in aircraft structures does not require disassembly for deburring, removal of chips, or cleaning to remove contamination from coolants or lubricants, saving time [34]. Orbital drilling with industrial robots creates lower axial cutting forces compared to conventional drilling and, therefore, allows the use of low-cost standard industrial robots for drilling holes within the required hole tolerances [27]. The chip evacuation is a benefit of helical milling since they are conveyed off the cutting zone by the radial clearance between the hole and the tool, while in drilling they are evacuated through the flute space [3]. Consequently, air blow can be used to assist chip transportation, avoiding wet machining merely for chip evacuation [3]. Minimum quantity lubrication (MQL) can improve geometrical quality, reduce temperature and force levels [26,35]. Counterboring and countersinking are also facilitated in helical milling by just programing the correct tool path [3]. Good fluid conditions are also attested in helical milling [35–37]. Apart from drilling, wherein difficult-to-cut materials the tool failure mode is predominately catastrophic, in helical milling, the evolution of tool wear can be monitored and the wear mechanism can be better understood [3,38]. In helical milling, tool wear can be monitored in peripheral cutting edges, where the contact between tool and workpiece material is not constant; and in bottom cutting edges, where the cut is continuous and more severe wear is generated, defining the tool life criteria [26,38]. By adjusting the

eccentricity, the tool path can be corrected to compensate tool wear, preventing borehole diameter deviations [37,39]. Helical milling studies were performed to obtain cutting conditions to achieve a chatter-free process [36]. Helical milling presents great potential in machine composite materials due to temperature and cutting forces reduction [1,9,25]. Consequently, helical milling presents lower delamination and damage in machining of CFRP [9,25,40]. 2. HELICAL MILLING KINEMATICS Regardless some confusion between helical milling and orbital drilling [29,32], these similar processes are not exactly the same in relation to equipment used [41]. In the orbital drilling process, the cutting tool is fastened on a tool rotation spindle eccentrically from a planetary revolution spindle and it rotates independently [41]. The orbital drilling can be executed by robotic orbital drillers and by orbital drilling units [10,12,32,42]. In the helical milling process, executed by machining centres, the helical path is generated by the axial feed of CNC z-axis synchronised with circular motion generated by numerical interpolation of x and y axes, besides tool rotation around its own axis [32,41]. The circular movement of tool centre point (TCP), performed by interpolation of two linear axes, demands high feed drive acceleration, rigidity, and stiffness of the machine tool [9,10]. Helical milling is more difficult to be employed in aircraft assembly system due to the complex aircraft structures and small-size device access space [32,42]. However, the holes machined by orbital drilling have higher roundness than holes machined by helical milling [41,43]. There are several studies on orbital drilling devices development and tests [44–47]. As there is not a general agreement in this nomenclature, in this paper the two terms will be used, helical milling and orbital drilling, according to the choice of the authors. To describe the helical milling kinematics, it is important to standardize the nomenclature of the helical milling parameters. There are several nomenclature definitions in the literature, making it difficult to understand the calculations and to apply the optimal helical milling parameters achieved in the investigations. In this sense, it is important for the researchers and practitioners who are working with helical milling to have the same perspective, facilitating the understanding of the process and the building knowledge in the helical milling field. To describe the motion of the tool it is important to define two coordinate systems: the workpiece coordinate system and the tool coordinate system [2,6,9,33,48]. The workpiece coordinate system presents fixed coordinate axis direction x, y and z, while the tool coordinate system changes with tool rotation and orbital revolution. The workpiece coordinate system is the reference for helical milling kinematics. The kinematics of helical milling can be described by decomposition of the helical path [1,9,28,36,48]. The helical feed velocity of the helix (vf) in [mm/min] can be decomposed in the tangential feed velocity of the helix (vfht) in [mm/min] and in the axial feed velocity of the helix (vfha) in [mm/min] which, respectively, describes a circular motion on xy CNC plane and a linear motion on z-axis direction. This vector sum is expressed in Equation 1. In the linear trajectory, the axial feed velocity can be described considering the axial feed per tooth (fza) in [mm/tooth], the number of teeth (z) and the spindle rotation speed (n) in [RPM], as expressed in Equation 2. In the circular trajectory, the tangential feed velocity (vfht), which is the circular velocity of the TCP related to the helical diameter (Dh) in [mm], can be described considering the tangential velocity vft in [mm/min], related to bore diameter (Db) in [mm], as in Equation 3. The tangential velocity (vft) is mathematically expressed in function of the tangential feed per tooth (fzt) in [mm/tooth], the number of teeth (z) and the spindle rotation speed (n), as in Equation 4.

v f  v fha  v fht 2

2

(1)

v fha  f za  z  n

v fht  v ft

(2)

Dh Db

(3)

v ft  f zt  z  n

(4)

As vf is the helical feed velocity of TCP, related to Dh, generally used to setup the CNC program, it is important to calculate the helical feed velocity of the periphery vfp in [mm/min], applied when using radius compensation and also to know the maximum chip load in the peripheral cut. The helical feed velocity of the periphery vfp is expressed in Equation 5. The helical feed velocity of the periphery vfp can also be expressed as a function of chip load or feed per tooth in [mm/tooth], as in Equation 6.

v fp 

Db  vf Dh

(5)

v fp  f z  z  n

(6)

The axial cutting depth (ap) in [mm] in helical milling process depends mathematically on the tangential and axial feed velocities. Firstly, it can be expressed the helix angle α in Equation 7, and subsequently, in the Equation 8, the maximum axial cutting depth (ap*) in [mm/rev], which consists of the path of the helix. The maximum axial cutting depth ap* can also be described considering the axial and the tangential feed per tooth, fza, and fzt. Figure 1 illustrates the helical milling kinematics. According to Iyer [49], in borehole generating, the radial cutting depth (ae) in [mm] is measured in the xy plane as the ratio between the total area to be removed and the circular path length, as displayed in Equation 9, depending only on the borehole diameter and helical diameter. However, other authors consider that ae approaches the tool diameter Dt [2,36,50]. Considering that helical milling can be applied for borehole generating and for borehole enlarging [2,50], for borehole enlarging ae is expressed in Equation 10, as the height of tool contact arc, considering the final borehole diameter Db and the initial borehole diameter D0 in [mm].

v    arctan fha   v fht  a p  tan     Dh  *

ae  ae 

  Db2 4



(7)

f za    Db f zt

D2 1  b   Dh 4  Dh

Db2  D02 4  Dh

(8)

(9)

(10)

Figure 1. Helical milling kinematics ([9,51] with permission from Elsevier, license number: 4000270610594)

Another perspective to describe the helical milling kinematics is in relation to the circular motion and angular velocities [8,30,33]. Considering the spindle rotation speed (n) the angular velocity and the orbital rotation speed (no) also in [RPM], it can be defined the angular velocities of the tool rotation motion (ω) and of the orbital feed motion (ωo), both in [rad/s], in Equations 11 and 12, respectively. The orbital rotation speed (no) can be obtained considering the axial feed velocity (vfha) and the axial cutting depth (ap*) or considering the tangential feed velocity (vfht) and the helical diameter (Dh) as in the Equation 13. Another important characteristic which can be defined to describe the helix diameter and, consequently, the other parameters, is the eccentricity (e) in [mm], which is the distance between the borehole centre point and the TCP, calculated in Equation 14.



2n 60

(11)

o  no  e

2no 60

v fha a

* p



(12)

v fht

(13)

  Dh

Db  Dt 2

(14)

t

Figure 2. Undeformed chip dimensions ([9,51] with permission from Elsevier, license number: 4000270610594)

t

Figure 3. Undeformed chip geometry ([9,51] with permission from Elsevier, license number: 4000270610594)

The helical milling process consists simultaneously of a peripheral cut on the peripheral cutting edge with a discontinuous cut, which is similar to milling, and a frontal cut on the axial cutting edge with a continuous cut, which is similar to conventional drilling or plunge milling. The undeformed chip geometry was

described in helical milling process [9,48,51]. Regarding this description, considering the engagement angle φ in the peripheral cut, the axial cutting depth ap increases (decrease) from 0 to ap* (from ap* to 0) in downcut (upcut). The undeformed chip thickness htan presents a sinusoidal behaviour with maximum equals to fzt, regarding the engagement angle φ, as illustrated in Figure 2. The chip is discontinuous as illustrated in Figure 3. Considering the axial cut, the cross section remains constant over the engagement angle, the undeformed chip thickness hax is equal to the axial feed per tooth, as in Figure 2. Figure 3 illustrates the undeformed chip geometry. These geometrical relations are:

htan  f zt sin 

(15)

hax  f za

(16)

Dt 2

(17)

bax 

The cross section of the undeformed chip was also approximated [9,48,51] considering large ratios of tool to borehole diameter (Dt/Db), in relation to the tool rotation angle (φ). The cross section depends on the axial cutting depth ap(φ), calculated in relation to the tool rotation angle (φ) as in Equation 18.

a p   

a*p 180



(18)

The authors derived a model for ap to approach low ratios Dt/Db. For 0 ≤ Dt/Db ≤ 0.5, a borehole enlarging operation is obtained, while for 0.5 ≤ Dt/Db ≤ 1, a borehole generation in only one operation is achieved. For low Dt/Db ratios, the cutting depth is divided and calculated considering the auxiliary angle (ψ) and considering the helix rotating angle (ξ). Then, these calculations can be resumed in Equation 19. The angles ψ and ξ are dependent on the route PsMb and on the tool rotation angle (φ). These quantities can be determined based on the geometric relationships shown in Figure 4 [48].

a p  a p , 0    a p    a*p

 360

 a*p

 360

 a*p

 

(19)

360

A Ps,0

Ps,0

Tool

Ps,1 β

Mt,1

Mt

Dh

Db

Dt

φ

Mt,0

ψ

Mb

Ps,n

ξ Mb

A

Linha auxiliar

Borehole

(a)

Tool perimeter Auxiliary line

Helical path

Borehole

(b)

Figure 4. Geometrical relations to define the undeformed chip geometry in helical milling (Adapted from [48], with permission from Wiley, license number: 4000320554465)

Considering these geometric definitions and the undeformed chip geometry, the volumes of tangential (Vtan) and axial cutting (Vax), and the ratio between these volumes (K) were defined. These definitions can be resumed in Equation 20. The final relation for the ratio K is only dependent on the borehole and tool diameters [48].

1 * * a f D Vtan 2 p zt t 2a p f zt 2Db  Dt  K    1 2 Vax Dt f za Dt Dt f za 4

(20)

The ratio between the peripheral cutting edges and frontal cutting edges volumes was described considering an arbitrary inspection radius Ri [1]. Consequently, it can be determined the peripheral cutting depth ap(1i) and the front cutting depth ap(2i) and the total cutting depth will be the sum of these two parts ap* = ap(1i) + ap(2i). The arbitrary inspection radius Ri, in the workpiece coordinate system, which varies from 0 to Db/2, describes a circumference Ui with 2π rad and comprises only a part of the tool perimeter with a circumference Uti and a 2β rad. There is a region delimited by the radius Rpfc = Dt/2 – e in which pure frontal cut occurs analogously to drilling or plunge with an eccentric movement. With Ri < Rpfc, ap(1i) = 0 and ap(2i) = ap*. There is a point with R = 0 in which the vc = 0 as in drilling, due to frontal cut. Therefore, extrusion rather than cut can occur in this region. It can be stated that Ri varies from 0 to Dt/2 and the calculation of the periphery cutting depth ap(1i) and the front cutting depth ap(2i) depends on Db, Dt, ap* and Ri values, as depicted in Equations 21 and 22. Figure 5 presents the geometrical parameters of this approach.

*

a p 1i 

*

a p 2i 

   1   i  *  a p 1    a p 1  arcsin          *

 *  i a p  ap    arcsin      *

   

   





    

D 2 D D 2 Ri 2  t  b t Dt 2 2 2  2 Db  Dt



 

D 2 D  Dt 2 Ri 2  t  b Dt 2 2 2  2 Db  Dt

 

2

 

2

  Ri    

  Ri   

(21)

(22)

The ratio between frontal and peripheral cut (G) was also calculated considering the approach of the arbitrary inspection radius Ri. Figure 6 presents the cut volume of the frontal and peripheral cut. The ratio G between the volume removed by the peripheral cutting edge and the volume removed by the frontal cutting edge is independent of the process velocities and depends only on the borehole and tool diameters as depicted in Equation 23 [1].

G

Vtan Vax

 Db 2  Dt 2    a p*    2 2 2  2 Db  Dt     2 2 Dt  Dt  *     ap  2 

(23)

The ratio of the frontal to peripheral volumes, considering the approach of the engagement angle (φ) K, and the approach considering the arbitrary inspection radius (Ri) – G were compared [37]. The results are resumed in Table 1 and Figure 7, besides, the linear relationship in Equation 18 was considered in the comparison. The analytical model from Brinksmeyer et al. [1,37] reported the same solution of the exact solution of Denkena et al. [48], while the analytical approach proposed by Denkena et al. [48] reported

significant difference in lower ratios k. The correct results, Equations 23 and 18, can be verified through a CAD modelling of the peripheral and frontal cut. As these models do not depend on the helical milling velocities, but only on the diameters, the cutting tool selection plays an important role on the process performance. The lower the diameter ratio k is, the higher will be the volume ratio G, approaching the helical milling of the milling process, with high material removal rate by peripheral cut.

ηi

URi Uti

ap(1i)* ap ap(2i)*

Uti URi

ap

URi

βi

Uti

Db

Dh

Dt Figure 5. Dependency on the cutting zones (ap(1i)*, ap(2i)*) on the arbitrary inspection radius (Ri) on the tool path (Adapted from [1], with permission from Springer, license number: 4000210981914) Table 1. Comparison of the results regarding the volume ratio ([37], with permission from Wiley, license number: 4010230598121) Diameter ratio k 0.5 0.6 0.7 0.8 0.9 1

Eq. 23 3.00 1.78 1.04 0.56 0.23 0.00

Volume ratio K Eq. 20 2.00 1.33 0.86 0.50 0.22 0.00

Eq. 18 3.00 1.78 1.04 0.56 0.23 0.00

FIRST VIEW Front cutting Peripheral cutting edge volume edge volume

SECOND VIEW Front cutting Peripheral cutting edge volume edge volume

Drilling and milling volume

Drilling and milling volume

Figure 6. Volume of frontal cut and peripheral cut in different views ([1], with permission from Springer, license number: 4000210981914)

3.0

Volume ratio K = Vax/Vtan

Milling

2.5 Exact solution

2.0

Drilling

1.5

Linear approach

1.0 Error

0.5 0.0 0.5

0.6

0.7 0.8 Diameter ratio k = Dt/Db

0.9

1

Figure 7. Influence of the diameter ratio k on the volume ratio K (Adapted from [48], with permission from Wiley, license number: 4000320554465) 3. CUTTING FORCES IN HELICAL MILLING Knowledge of the behaviour and magnitude of cutting forces is very important to estimate cutting power and to obtain tight tolerances and low levels of tool wear. Appropriate prediction of the force components collaborates with the correct choice of the cutting parameters, to avoid machine tool vibrations, improve workpiece surface quality, geometrical accuracy, to guarantee process stability, and also to analyse the dynamic performance of spindle units [8,52].

The undeformed chip geometry of the helical milling process may be used to explain the impact of the axial and tangential feeds per tooth on the process forces, and also to model the cutting forces using a semianalytical approach [9,53]. The periphery cutting edge produces mainly radial cutting force while the front cutting edge produces not only radial cutting force, but also axial cutting force. The radial cutting force may lead to a radial deflection of the tool and cause vibrations, while the axial cutting force may deform the workpiece in the axial direction, leading to poor surface finishing [54]. The influence of the cutting speed gradient along the cutting edges cannot be taken into account in analytical modelling of cutting forces, while it is important in end-cutting operations such as drilling or helical milling. Moreover, the cutting mechanisms of orbital drilling, especially under the tool or at the edge corner, cannot be assumed as orthogonal or oblique cutting phenomena. Then, semi-analytical approaches, also called mechanistic approaches, have been developed [53]. Tool’s geometrical features and the tool-workpiece interaction taken into consideration in cutting force models may be useful for chatter modelling [55]. Studies about cutting forces in helical milling are generally carried out using two coordinate systems: the workpiece coordinate system, which is fixed with origin in the borehole centre, and the tool coordinate system which moves along the circular path [6,9,52,56,57]. The cutting forces components in the workpiece coordinate system are the orthogonal components Fx, Fy and Fz and can be monitored with a stationary dynamometer [29,33]. In the tool coordinate system, different approaches may be applied. Some authors use to characterize the tangential force (Ft) in the opposite direction of the cutting direction, the radial force (Fr) acting toward the centre of the tool, and the axial force (Fa) along the z-axis, as illustrated in Figure 8 [6,52]. The axial force is equal the Fz component of the workpiece coordinate system, Fa = Fz, and the radial force, also denoted as resultant force, may be obtained as the resultant of the components Fx and Fy of workpiece coordinate system, Fr = (Fx + Fy)1/2 [5,33,38]. The tangential force and the radial force may also be referred as feed force (Ff) and feed normal force (FfN), respectively [9,51]. Several cutting force models for helical milling have been proposed. For instance, a mechanistic cutting force model for helical milling of CFRP considering the fibre cutting angle was established according to cutting principle of helical milling. Non-linear cutting force coefficients were predicted by using the response surface methodology (RSM). The mechanistic model presented 15 % error over validation tests [6]. A dynamic cutting force model was proposed for helical milling, in which the cutting mechanism and the cutting force contribution to both the peripheral and the front cutting edges are taken into consideration simultaneously. Simulations and tests under different cutting conditions in aluminium Al 7075-T6 were conducted to reveal the impact of fzt and fza on the cutting force components in the workpiece coordinate system [29,36].

Fa Fy

vfht

Fx

Fr Ft

n tool

(a)

(b)

hole

Figure 8. Cutting force components on (a) workpiece coordinate system; (b) tool coordinate system (Adapted from [4,53], with permission from Elsevier, license numbers: 4000221427285 and 4000350097935)

A cutting force model was established to predict the cutting forces and torque in helical milling as a function of helical feed, spindle velocity, axial and radial cutting depth and tool geometry. The forces both on the side and on the end cutting edges along the helical feed path were described by considering the tangential and the axial motion of the tool. The dual periodicity which is caused by the spindle rotation, and the period of the helical feed of the cutting tool, has been included. Experimental confirmation was performed in Ti-6Al-4V and about 10% error was achieved considering the simulation results [2]. Force prediction in orbital drilling was realized considering the frontal and peripheral cut. These cutting portions were simulated individually with two different force models using a set of milling and drilling trials to obtain the specific cutting force coefficients, and the superposition of the models resulted in the prediction of the force components in orbital drilling. Tool deflection due to feed normal force was verified causing dimensional deviation of the borehole diameter. The models were proposed to compensate for this error [9]. A mechanistic cutting force model was developed considering the tool geometry and cutting parameters. The model considers the instantaneous chip form which is dependent on the tool geometry and cutting parameters, so that the chip load may be controlled to improve the borehole quality [53]. A fracture mechanics-based model was presented for predicting the critical axial force causing the onset of crack propagation and delamination during orbital drilling. The main factors that reduced the risk of exit delamination in CFRP were explained in terms of the eccentric distributed axial load applied by the tool, and the shift of the principle work done by the cutting tool towards the tangential direction [25]. Figure 9 shows a point load acting at the centre of a circular plate in the case of conventional drilling, versus the eccentrically

distributed load acting along the cutting lips of an end milling tool on a circular edge-clamped plate in orbital drilling case [25,58].

(a)

(b) Figure 9. Axial force causing the onset of exit delamination (a) conventional drilling; (b) orbital drilling (Adapted from [25,58], with permission from Elsevier, license numbers: 4000281291748 and 4000351025274)

The optimisation of cutting force and its respective effect in delamination during orbital drilling of CFRP was demonstrated showing that the damage and the tear length increased with thrust force. The cutting force was reduced by 26% and it was observed burrs in the entrance, delamination, and tear in the exit of the borehole before optimisation, and low damage in the borehole with optimized parameters. However, the authors did not detail the optimisation method [59]. Figure 10 shows the simulated and the measured cutting forces in the helical milling of Ti–6Al–4V after being filtered at a small time window considering different cutting parameters. The mechanistic models presented approximately 10% errors. These errors, according to the authors are due to some noise interference in the measurements, the assumptions that the tool and workpiece are rigid, that there is no tool deflection and vibration, and the effect of the tool run-out which was not considered in the modelling process, which could affect the level of the cutting force components [8]. A force sensorless method was proposed based on cutting force observer for monitoring the thrust force and identifying the drilling material during the orbital drilling of CFRP-Ti stacks. The cutting force observer, which is the combination of an adaptive disturbance observer and friction force model, is used to estimate the thrust force. An in-process algorithm was developed to monitor the variation of the thrust force for detecting the stack interface between the CFRP and titanium materials. Robotic orbital drilling experiments have been conducted to evaluate the method on CFRP-Ti stacks [60]. Different strategies of helical milling of CFRP were evaluated. As the number of machined holes increased, the thrust force increased according to the progression of tool wear. By applying ultrasonic vibration, the thrust force was reduced compared with conventional helical milling because of the reduction in friction between the cutting tool and the workpiece. The thrust force levels in dry machining were the smallest. This was

because the epoxy resin of the CFRP was softened by the cutting heat. The thrust force was also reduced by applying the proposed cryogenic tool cooling [61].

(b)

(a)

Figure 10. Comparison of measured and simulated cutting forces (a) n = 3200 rev/min, fz = 0.05 mm/tooth and ap* = 0.2 mm/rev; (b) n = 3700 rev/min, fz = 0.065 mm/tooth and ap* = 0.1 mm/rev ([8], with permission from Springer, license number: 4000261504362) 4. TEMPERATURE IN HELICAL MILLING High cutting temperature can lead to serious tool wear, which induces the decrease of the machining accuracy [62,63]. In conventional drilling, most of the mechanical energy associated with chip formation is converted into heat. The resulting temperature rise may cause thermal damage to both the drill and workpiece, such as rapid tool wear and dimensional borehole errors. As the drill enters further into the workpiece while drilling and chips are produced in a confined space, the heat is readily stored in the cutting material inside the hole [64]. In the case of CFRP materials, the drilling-induced delamination and thermal damage are serious problems [58]. As a beneficial process in comparison to conventional drilling, helical milling process employment should support to generate boreholes with less ill effects due to temperature. For instance, the cutting heat dissipation was greatly improved in the helical milling of quartz fiber reinforced ceramic, with electroplated diamond trepanning tool [65]. To measure temperature in the helical milling of CFRP, it has been used thermocouples [4], Infrared camera [25,62], and thermograph [66]. Air cooling was used to cool the cutting point and to remove the chips, and slightly better results in terms of cutting forces and temperature were achieved [66]. However, it has been reported that cool fluid results in the bad performance of the CFRP machined, and the powder-like chip is bad to the machining centre in the air cool condition [4]. It was found that the cutting heat becomes lower by increasing revolution speed. Chips pulverised into small size reduces cutting temperature by absorbing cutting heat on the cutting of CFRP composite plate [66]. Air cooling reduced the temperature of the cutting point in the helical milling of CFRP by 30% when compared with dry machining [66]. The resulting cutting temperature in the helical milling of Ti-6Al-4V (200 °C) is much lower than the phase transformation temperature of Ti-6Al-4V (985 °C) [31]. The effect of cutting conditions in the maximum tool temperature measured at the instant of tool exit of helical milling of CFRP was investigated, as graphically depicted in Figure 11. It was observed a growth trend of temperature with the

increase in the axial feed velocity of the helix (vfha). In a different way, the increase in the revolution speed resulted in temperature reduction [25].

Figure 11. Effect of helical milling cutting parameters on temperature ([25], with permission from Elsevier, license number: 4000281291748)

A heat transfer model was developed to investigate the temperature distribution of CFRP workpiece during the helical milling process. The heat transfer model was tested experimentally and the results could be applied to optimize the cutting condition and limit the thermal damage in the CFRP workpiece [4]. The temperature in the orbital drilling of CFRP-Ti was compared with the temperature in the drilling of CFRP-Ti. Compared with the maximum temperature during traditional drilling, the maximum temperature during orbital drilling decreased more than 36.3%. While in drilling with or without vacuum dust removal system the cutting temperature generated was almost the same, in orbital drilling, the vacuum dust was responsible for reducing the temperature in cutting zone for 14%, because this removal system could directly take cutting heat and cutting chips away from the cutting area [62]. The temperature and the axial force in the orbital drilling were also compared to the temperature and axial force in the drilling of Al-CFRP-Ti stacks. It was shown that in both processes the average temperature and the average feed force increased significantly from drilling the aluminium, over CFRP, to the titanium layer. The average temperature in conventional drilling was higher than in orbital drilling. Specifically, in CFRP and titanium, the average temperature load duplicated when changing to conventional drilling. However, the variability of the measurements was graphically quite high. Then, to assure the difference with statistical significance, the authors should use statistical inference [67]. 5. WEAR AND TOOL LIFE IN HELICAL MILLING The understanding of tool wear mechanisms is important to improve the productivity and to reduce tool cost [68]. The progressive tool wear involves the reduction in the process efficiency caused by a rise in cutting forces and cutting edge temperatures. So, while tool wear increases, vibrations appear and reduce the finishing quality of generated surfaces [69]. Most part of the recent tool wear studies in helical milling are mainly focused in Ti-6Al-4V alloy [26,30,31,68,69] and CFRP [7,40,61]. Titanium alloys are ranked as difficult-to-cut material owing to low thermal conductivity, high chemical reactivity, low modulus of elasticity and serious work

hardening. Cutting velocity of processing titanium alloys is low and tool life is short [68,70]. In the machining of CFRP, the tool wear caused by the high hardness of carbon fibers must be considered [61]. Table 2 summarizes some recent tool wear studies in helical milling in relation to work material, tool material and coatings; wear monitoring techniques applied; cutting edges considered in tool wear investigation, since in helical milling the tool wear can be developed in the frontal and peripheral cutting edges; wear types; wear mechanisms; tool life criteria; and number of holes machined. In the investigation of the tool wear of coated tungsten carbide tools in Ti-6Al-4V alloy machining, two different coatings were compared. The diamond-coated tool presented shorter life than the TiAlN-coated tool since the machining with the TiAlN-coated tool is more stable and presented lower levels of cutting forces. In the end of tool life, the axial cutting force presented an increasing for both coatings [68]. In other investigation of a carbide TiAlN-coated tool wear during helical milling of Ti-6Al-4V, no diameter and roundness variations were observed with tool wear progression. The axial cutting force, burr height and roughness presented an increasing trend with tool wear progression [30]. In the helical milling of CFRP with a carbide ball end mill, the axial cutting force increased with tool wear progression [61]. In another work in CFRP, orbital drilling was compared with conventional drilling. Orbital drilling presented better performance in terms of delamination, borehole damage, borehole exit quality, and cutting forces. However, conventional drilling presented lower diameter deviation and higher productivity. In the orbital drilling of CFRP, after 800 holes, flank wear at the peripheral cutting edges removed the diamond coating [40], causing dimensional deviation. The effect of tool wear on delamination during helical milling of CFRP was verified [7]. Different coolant-lubricant conditions were tested in the helical milling of Ti-6Al-4V. The dry condition presented the poorest results in terms of tool life, cutting force and roughness. MQL presented the best results in terms of wear, exhibiting a distinct advantage in chip evacuation and postponing the wear progression [26]. Helical milling was compared with drilling of Ti-6Al-4V. Helical milling tool life was higher than the tool life in drilling. The wear in helical milling was gradual in contrast to the fracture occurred in the drill outer corner [31]. Helical milling and drilling of AISI D2 hardened steel were evaluated. The evolution of flank wear in helical milling was progressive and robust with attrition and microchipping in flank wear, unlike in drilling wherein tool failure was predominantly catastrophic [3]. Figure 12 presents scanning electron microscopy (SEM) images of a tool wear investigation case of helical milling of CFRP considering bottom and periphery edges. In Figure 12(a) at the frontal edge, the predominant wear mechanism in the flank face was abrasion as summarized in Table 11, while in Figure 12(b) the adhesion of CFRP powder was the principal wear mechanism. The tool wear in frontal cutting edges was more severe, defining the tool life [7].

(a) Figure 12. SEM images of tool wear state in CFRP (a) bottom edge; (b) periphery edge ([7], with permission from JSTAGE, grant received by email)

Figure 13 presents SEM images of tool wear in frontal edges in helical milling of Ti-6Al-4V. Chipping and adhesion were observed in the first stage of tool wear, posteriorly followed by flanking and crater.

Figure 13. SEM images of tool wear state in Ti-6Al-4V (a) chipping at cutting time of 12.5 min; (b) adhesion at the cutting time of 12.5 min; (c) flaking at the cutting time of 18.3 min; and (d) crater at the cutting time of 18.3 min ([30], with permission from Springer, license number: 4000290758460)

Table 2. Tool wear in helical milling

Paper

Qin et al. [68]

Li et al. [30]

Work Cutting material edge of / Tool Wear monitoring wear material technique investigati / on Coating Ti-6Al4V/ Tungste Scanning electron n microscope, digital carbide/ microscope and energyTiAlN; dispersive X-ray Diamon d

Ti-6Al4V/ Carbide tool/ TiAlN

Optical microscope, scanning electron microscope and energydispersive X-ray

Frontal and peripheral cutting edges and tool nose

Frontal and Stereoscopic optical peripheral microscopy, scanning cutting electron microscope and edges energy-dispersive X-ray CFRP/ Frontal ( Ishida et Carbide Scanning electron at end) al. [61] ball end microscopy cutting mill/edges CFRP/ Cement Frontal ed and Voss et al. carbide 3D microscopy peripheral [40] end cutting mill/ edges Diamon d Fernández Ti-6Al-Vidala et 4V/-/al. [69]

Wang et al. [7]

Qin et al. [26]

CFRP/ Carbide milling cutter/ TiAlN

Ti-6Al4V/ Solid carbide end mill/ CrN

Scanning electron microscopy and energydispersive X-ray

Digital microscope, scanning electron microscopy, scanning electron microscopy, energy-dispersive X-ray

Frontal and peripheral cutting edges

Frontal and peripheral cutting edges

Wear type

Flank wear

Flank (front, periphery and nose), crater (periphery) and catastrophic (nose) wear

Flank wear

Wear mechanisms

Tool life criteria

Number of holes machine d

88 holes (TiAlNcooted tool); 70 VB = 0.2 mm holes (diamon d-cooted tool) 12 holes (vc = 120 Chipping/fract VB(aver) = m/min); ure, diffusion, 0.2 mm, 40 hole and oxidation VB(max) = (vc = (frontal cutting 0.3 mm, 100 edges); excessive m/min); diffusion, chipping/flank 50 holes adhesion, tool ing or (vc = 80 failure (tool catastrophic m/min); nose) failure. 85 holes (vc = 60 m/min) Adhesion, diffusion, chipping (in the union of 10 holes frontal and peripheral cutting edges) Adhesive wear, oxidation wear, coating flaking and chipping

Flank wear

-

-

40 holes

Flank wear

Diamond coating flanking at peripheral cutting edges

-

1000 holes

Flank wear

Abrasion and coating flanking (frontal cutting edges); adhesion (peripheral cutting edges)

-

80 holes

Flank and corner wear

Microchipping, thermal cracking, abrasion, adhesion, chemical erosion and flaking

VB(aver) = 0.2 mm, 40 holes VB(max) = (dry), 0.3 mm, 145 corner wear = holes 0.2 mm, (flood) chipping = 0.2 and 160 mm or holes catastrophic (MQL) failure

Ti-6Al4V/ Digital camera, optical Frontal Ultramicroscopy, scanning and Zhao et al. fine electron microscope and peripheral [31] grain an energy dispersive Xcutting carbide/ ray edges TiAlN

Iyer et al. [3]

AISI D2 hardene d steel/ ball nose insert; flat nosed solid carbide end mill/ TiCN; TiN and TiAlN

-

-

Flank and crater wear

Flank wear

VB(aver) = Adhesion, and 0.2 mm, chipping VB(max) = (frontal cutting 0.3 mm, edges); excessive Flanking chipping/flank (peripheral ing or cutting edges) catastrophic failure.

Attrition and microchipping

VB = 0.3 mm

110 holes

10 holes (TiCNcoated inserts); 16 holes (TiAlNcoated inserts)

6. STATISTICAL AND SOFT COMPUTING TECHNIQUES ANALYSIS, MODELING, AND OPTIMISATION IN HELICAL MILLING Statistical analysis can indicate the most influential helical milling parameters, calculate its effects and the statistical significance in the responses of interest. Unlike the one-factor-at-a-time approach, the design of experiments (DOE) allows for the experimenter to evaluate not only the main effect but also the interaction among cutting parameters. Besides, with the appropriate experimental design, a regression model may be obtained to predict the response in a specified experimental space. In helical milling process there are only some initial investigations using statistical modelling and optimisation techniques. Factorial designs have been applied to investigate the effect of helical milling parameters in CFRP [6,7,22,71], to calculate cutting force coefficients [6] and to design the experimental planning to modelling the delamination factor in CFRP [22]. Taguchi design was applied in the to study the effects of process parameters including; cutting speed, feed rate, axial depth of cut and workpiece hardness on dimensional and geometrical tolerances in the helical milling of AISI 4340 steel [72]. Also, L9 Taguchi arrays were used to study the effect of helical milling parameters in roughness and roundness of helical milling of AISI 1045 steel, and to investigate the axial force in the helical milling of aluminium [43,50]. Response surface methodology (RSM) was applied to the modelling of mean surface roughness (Ra) in the helical milling of die-steel. The second-order model was validated with confirmation runs achieving error levels less than 7% [73]. To fulfil a mechanistic force model for helical milling of CFRP, RSM was applied to estimate the cutting forces coefficients reaching prediction capability equal to R2pred = 92.45% [6]. Using the data obtained from full factorial designed experiments, the significance of the process parameters and its effects in the delamination factor was estimated through an artificial neural network (ANN) model. MATLAB ANN Toolbox was used for modelling. The best-fitted model achieved a fitting correlation of

the developed prediction model R = 0.9843. Figure 14 shows some interactions between the cutting parameters estimated by the ANN model in the delamination factor of helical milling of CFRP [22].

n (rev/min) n (rev/min)

ap*

f zt (mm/rev)

(e)

ap* (mm/rev)

(b)

ap* (mm/rev)

(c)

(a)

n (rev/min)

f zt (mm/rev)

(mm/rev)

n (rev/min)

f zt (mm/rev)

(d)

f zt (mm/rev)

ap* (mm/rev)

(f)

Figure 14. Interaction effects caused by the feed rate and screw pitch on the delamination factor (a) fzt = 0.02 mm/tooth; (b) fzt = 0.04 mm/tooth; (c) ap* = 0.1 mm/rev; (d) ap* = 0.25 mm/rev; (e) n = 4000 RPM; and (f) n = 8000 RPM ([22], with permission from Springer, license number: 4000280313837)

Linear regression with a log transformation was applied to describe the effect of cutting velocity (vc) and orbital speed (no) to predict the axial cutting force component Fz in the helical milling of die steel Cr12 [74]. Edge cutting components to predict cutting force were obtained through linear regression [8]. Linear regression was also applied to predict the delamination in helical milling of CFRP in relation to tangential force [57]. In the force observer used to monitor the thrust force and detection of stack interface for CFRP/Ti during the orbital drilling process, the changes in the force observations were identified by a moving linear regression algorithm [60]. ANOVA was used to investigate the significance of the effects of cutting parameters in roughness and roundness in helical milling of AISI 1045 steel [50] and to investigate the significance of the effects of cutting parameters in cutting force, temperature, roughness, and hole size in helical milling of CFRP [71]. Ant Colony Optimisation (ACO) was applied to machining path optimisation of helical milling to define the hole-making sequence of several boreholes converted to a Travelling Salesman Problem, decreasing the tool path length in 41%. However, no optimisation was applied to the helical milling cutting parameters itself [75]. The evolutionary multi-objective optimisation of competing parameters of the helical milling process of CFRP, was realized through hybridizing Kriging as a meta-modelling technique and genetic algorithm. The trade-off space among the competing objective functions was evaluated and the productivity was defined as criteria to choose the best optimal point among the set of points which represents the trade-off space [71]. Multiobjective robust optimisation of the eco-friendly helical milling of Al 7075 was realized considering cutting force, roundness and material removal rate, to achieve sustainable Pareto optimal solutions through the augmented-enhanced normalized normal constraint method. Robustness was considered with regard to tool

overhang length variation. The trade-off between these sustainable outcomes was evaluated and a global solution was achieved through the technique for order of preference by similarity to ideal solution (TOPSIS) [11]. Finite element analysis was developed using a 3D finite element model for helical milling of titanium alloy Ti-6Al-4V. The proposed model takes into consideration the damage beginning and evolution in the workpiece material, and a contact model at the interface between end-mill and workpiece was obtained [20]. 7. BOREHOLE QUALITY IN HELICAL MILLING With helical milling, boreholes can be obtained with only one operation, eliminating subsequent machining processes such as re-drilling, reaming and countersinking [19]. Hole quality analysis includes dimensional, geometrical and microgeometrical deviation. Moreover, burr, delamination, and other characteristics are important aspects to be analysed in machining of holes. In this section different works considering dimensional, geometrical and microgeometrical aspects in helical milling were summarized. In the case of orbital drilling applications in aircraft structures, the requirements are mainly dictated by the need to join two metallic sheet layers (package). The hole must attain geometric and position specifications in both sheets, according to each location in the aircraft. For example, circularity error (or roundness) might be less than 26.0 µm, position error less than 0.5 mm, and perpendicularity deviation less than 0.5° [12,21,76]. 7.1. Dimensional quality Table 3 presents some studies dealing with dimensional results in the helical milling process. These works are about helical milling in CFRP [7,40,77,78], Ti-6Al-4V [19,79], CFRP-Ti stacks [5] and AISI D2 hardened steel [3]. Depending on the tool stiffness the cutting forces in the feed and orthogonal directions lead to tool deflection and consequent dimensional deviation [19]. In the case of CFRP-Ti stacks, the variation of the diameter may be due to the increased tool wear when processing the bottom Ti plate. The diameter fluctuations are relatively large in the exit of CFRP and entrance of titanium alloy. For laminated materials, hole diameter will change when machining from one into another material because that processing characteristics of two materials are different and the jointing face will impact the tool wear state; the material thickness will influence the temperature of cutting zone causing severe tool wear [5].

Table 3. Dimensional error in helical milling Paper

Measurement system

Work material

Db – nominal [mm]

Deviation [µm]

Resulted Tolerance

Wang et al. [77]; Wang et al. [7]

Micrometer

CFRP

10

±10

IT6

Ti-6Al-4V

10

CFRP

6.35

CFRP

12

27 to 43

IT8 - IT9

Wang et al. [79] Voss et al. [40] Sultanaa et al. [78]

Micrometer for inside measuring Coordinate measure machine Coordinate measure machine

Hole entry: 4 to 18; hole middle: 10 to 32 14.6 (first 800 holes); 39.1 (over 1000 holes)

IT6 - IT8 IT7 - IT10

Wang et al. [5]

Coordinate measure machine

CFRP/Ti

10

Single layer: 5 to 25 (Ti), -10 to 9 (CFRP); Stack: -50 to -15 (Ti), -30 to 10 (CFRP)

IT9 - IT10 (stack)

Iyer et al. [3]

-

AISI D2 hardened steel

16

-

IT7

Olvera et al. [19]

Coordinate measure machine

Ti-6Al-4V

9

Eguti and Trabasso [12]

Coordinate measure machine

Al7075-T6

4.77

Hole entry: 21 (BHM), 53 (CBHM); hole exit: 35 (BHM), 79 (CBHM) Hole entry: -2.5 to 2.1; Hole exit: -4 to -1

IT8 - IT11 IT5

In the helical milling of CFRP stacks of 68 unidirectional prepreg fibre layers with epoxy resin as matrix, after 800 holes a heavy decrease in diameter measured values caused by the failure of the diamond coating on the peripheral cutting edges [40] was noted. In the multi-objective optimisation of orbital drilling of CFRP, this process outperformed the drilling process in most of the results, except for the hole size errors [71]. In helical milling of AISI D2 hardened steel using the solid carbide tool, the dimensional tolerance of holes was in the band corresponding to IT7 quality [3]. The dimensional deviation in the helical milling of CFRP-Ti compound was investigated and a significant diameter deviation in the transition between CFRP and Ti layers was achieved. Also, an increase in the axial feed per tooth accompanied by higher process forces resulted in a reduction of the bore diameter in the CFRP layer as well as in the titanium layer [9]. The helical milling was compared with the conventional drilling of the aluminium alloy A5052. Helical milling presented lower dimensional deviations than drilling. Besides, MQL and flood coolant lubrication conditions presented better results in terms of dimensional deviation than air blow condition [35]. The orbital drilling to obtain boreholes with 4.77 mm (3/16”) in Al 7075-T6 aluminium alloy with a solid end mill, Dt = 3mm, with consideration to dimensional deviation was studied. The measurements were done considering the hole position (1 - 7), for different feed velocities. It was obtained 26 µm tolerance range, (±13 µm), centred in the average diameter of each feed rate series. Besides, the holes in the entry presented higher diameter than the holes in the exit, and the boreholes obtained with vf = 50 mm/min presented results closer to target [12]. The difference in dimensions between entry and exit were also reported in the helical milling of Titanium alloy Ti-6Al-4V due to tool deflection directly related to the cutting forces increase, mainly caused by the rising of the contact length between the tool and the workpiece as the helical milling process moves forward [19]. The helical milling was applied to compensate the borehole diameter error in Cr12MoV hardened steel. The target diameter was Db = 10mm. It was shown that the errors in the entry decreased from 26.0 µm to 5.9 µm for Dt = 8 mm and from 67.5 µm to 3.0 µm for Dt = 6 mm, respectively, if using a compensation approach. For borehole exit, the hole form errors were averagely decreased from 40.8 µm to 3.5 µm for Dt = 8 mm and from 72.0 µm to 3.7 µm for Dt = 6 mm, respectively [39]. 7.2. Roundness Roundness (or circularity) together with positional and dimensional deviation can difficult the assembly process in the aircraft industry. The roundness error was studied in the helical milling of A5052 aluminium alloy [35]. Roundness error of helical milling was lower than roundness error in conventional drilling. In this study, the lubrication effect in roundness was also studied. The dry results presented high roundness error, while MQL presented similar levels of roundness error of wet lubrication strategy. Table 4 summarizes some experimental results about roundness in helical milling. For instance, two different strategies were compared of helical milling of CFRP-Ti stacks [38]. The strategy I used the same

helical milling parameters to machine the two stacks, while strategy II used a set of parameters levels for CFRP and another different set for Ti-6Al-4V. Strategy II presented lower roundness error than strategy I and the roundness error in Ti plate was lower than the roundness error in CFRP plate. The roundness error in helical milling was compared considering indexable tool with solid carbide tool in AISI D2 hardened steel, as in Table 4 [3]. A significant aspect that can be observed in roundness measurement of boreholes obtained by orbital drilling is the error due to the backlash of the machine tool generated during the circular interpolation movement. It may occur in the moment that the machine tool reverses the direction of displacement of the axes [50]. Different hole making methods were compared in high-speed condition to analyse the stress distribution through the hole-drilling method. With this method, the obtained boreholes are used to measure the residual stress. Conventional drilling and orbital drilling considering different tools were compared to obtain boreholes with Db = 1.6 mm and the orbital drilling with common used six-blade bits, Dt = 0.8 mm, resulted in the best compromise of an ideal cylindrical hole and centricity to the centre of the strain gage rosette [80].

Table 4. Roundness error in helical milling Paper

Measurement system

Zhou et al. [81]

-

Fang et al. [32]

-

Iyer et al. [3]

-

Gaiyun et al. [38] Haiyan and Xuda [77]

Costa and Marques [50]

Li et al. [30] Wang et al. [7] Sultanaa et al. [78] Eguti and Trabasso [12]

Work material TC4 Ti alloy CFRP AISI D2 hardened CFRP-Ti stacks

Coordinate measure machine Coordinate measure CFRP machine Roundness measurement system AISI 1045 - rotational datum steel method Coordinate measure Ti-6Al-4V machine Coordinate measure CFRP machine Coordinate measure CFRP machine Coordinate measure Al7075-T6 machine

Db - nominal [mm] 15 13 16 10

Roundness [µm] Hole entry: 7; Hole exit: 67 7 25 (indexable tool); 10 (solid carbide mill) CFRP: 21.4 to 25; Ti-6Al4V: 17.8 to 19.4

10

12 to 22

35

15 to 23 (roughing); 13.5 to 16 (finishing)

10

11 to 25

10

10 to 20

12

20 to 30

4.77

Hole entry: 11 to 30; Hole exit: 15 to 42

7.3. Roughness The performance quality of a manufactured part is determined, in part, by its surface quality resulting from the manufacturing process. The fatigue life of products undergoing cyclic loads such as aircraft parts, nuclear reactors, or automobile parts is highly affected by its surface integrity and quality. Clearly, the machining process affects the workpiece on resistance to fatigue, creep, and stress corrosion cracking [28]. Surface roughness levels obtained in experimental helical milling studies are summarized in Table 5. Higher levels of roughness were obtained in the helical milling of CFRP due to the physical form in a state of layered or laminates [82].

Together with workpiece material, many parameters present influence on surface roughness, such as tool accuracy, workpiece hardness, tool geometry, and machining parameters [73]. For instance, for the titanium alloy Ti-6Al-4V, the roughness Ra varied from 0.2 to 0.7 considering cutting parameters and cooling-lubricant condition. The wet condition together with lower levels of fzt and ap* presented lower levels of roughness [26]. Table 5. Surface roughness levels in helical milling studies Paper Qin et al. [73] Zhao et al. [31] Sultanaa et al. [78] Rahim et al. [82] Qin et al. [26]

Measurement system Surface Roughness & Contour Machine

Work material Cr12 die steel (35 HRC)

Surftest

Ti-6Al-4V

Form Talysurf profilometer Surftest Form Talysurf profilometer

Roughness [µm] 0.40 to 1.56

CFRP

0.71 to 2.69 (considering tool wear evolution) 13 to 16.6 (superabrasive diamond tool) 1 to 3

Ti-6Al-4V

0.2 to 0.7

CFRP

Iyer et al. [3]

-

AISI D2 hardened steel

0.3

Costa and Marques [50]

Form Talysurf profilometer

AISI 1045 steel

0.43 to 2.12

7.4. Burr, cap, delamination and other aspects in helical milling The contamination of the internal space with caps, burrs, and chips in the manufacturing of aircraft’s closed structures is not acceptable resulting in risk of corrosion and electric breakdown. When significant caps and burrs are produced, two more working steps in the assembly are necessary for the removal of the burr and the cleaning of the closed structure. Cap and burr formation result from the deformation of the workpiece material near the bore exit, caused by the forces of the cutting process [83]. The helical milling and orbital drilling process present low thrust force levels allowing for burr-less and delamination-free drilling of laminated composite material. It minimizes the risk for part deflection when drilling in thin structures, and it facilitates automation using light equipment such as industrial robots, which are force-sensitive [34].

Figure 15. Burr height at the exit of hole versus number of holes for dry helical milling ([30], with permission from Springer, license number: 4000290758460)

In the dry helical milling of Ti-6Al-4V alloy, the burr height at the exit of the hole evolution due to tool wear was evaluated, as illustrated in Figure 15. In stage I, the value of burr height was in the range of 0 to 0.1 mm. The burr height in stage II was is in the range of 0.1 to 0.3 mm, and the discontinuous and slight burrs can be observed at the edge of holes. In stage III, as the number of machined hole passed the 36th hole, burr height increased rapidly and presented excessive values up to tool failure [30]. This trend was also confirmed by other authors [5]. Burr formation was investigated in the helical milling of primed clad aluminium 2024. It was shown that orbital drilling with a higher axial feed and an increasing cutting speed improves the burr formation and the lowest burr was found in up milling with minimum quantity lubrication [83]. The burr formation at the entry and exit of boreholes obtained in helical milling clearly presented better results when compared with burr formation in drilling in the A5052 aluminium alloy [35]. In helical milling of Cr12MoV hardened steel (60 - 61 HRC), less burrs were generated in lower cutting velocity levels [39]. In a single hole obtained by conventional drilling, the thrust force in the last peck cycle was so high as to precipitate material breakout at the hole exit (Fig. 16a), in contrast to helical milled holes (Fig. 16b) [3].

Figure 16. Material breakout at borehole exit of AISI D2 hardened steel: (a) conventional drilling and (b) helical milling ([3], with permission from Elsevier, license number: 4000220850523)

Significant less borehole exit deterioration, i.e., delamination or uncut fibres, and less borehole channel deterioration, i.e., fibre cracks, pull-out and bending, occurred in the orbital drilling of CFRP when compared with the same borehole quality aspects in drilling, as demonstrated in Figure 17 [40]. By using an artificial neural network (ANN) model it was demonstrated that a high spindle speed matched with the appropriate feed rate and screw pitch minimizes delamination in the helical milling of CFRP. Moreover, the range of the delamination factor, i.e., the ratio between the maximum diameter of the damaged area and the nominal diameter of the drilled hole, of the helical milling of CFRP was from 1 to 1.6 [22]. The inhomogeneous nature of composite materials may generate undesirable consequences during machining such as rapid tool wear, fiber pullout, surface burning and smearing, pitting and delamination. Delamination, in particular, is strongly dependent on the cutting force component normal to the stacking plane in unidirectional and multidirectional laminate composites [84]. The defects associated with the drilling of CFRP, e.g., delamination, matrix burnout, and fiber pullout, are of major safety and economic concerns to the aerospace manufacturers [25]. In the case of laminated stacks of electrical steel, helical milling presented better results when compared with conventional drilling and plunge milling, as it does not result in significant delamination or burr formation [85].

Helical milling or orbital drilling minimizes the delamination in CFRP. For instance, in the orbital drilling of CFRP with single layer diamond tools delamination was observed at the entrance side for 2.3% of the drilled holes and almost 99% of the drilled holes had no exit delamination. To avoid delamination in the helical milling of CFRP feed rate may be reduced in the entrance and in the exit of the borehole. Also, in the interface between CFRP-Ti stack, helical milling ensures a smooth entrance in the harder titanium material, avoiding chipping of the cutting tool [78]. Table 6 summarizes delamination factor results in experimental studies of helical milling of CFRP.

Figure 17. Comparison of micrographs at 1st, 400th and 1000th bore with critical fibre cutting angles ϕ = 90° and ϕ = 135° ([40], with permission from Elsevier, license number: 4000310025520)

Table 6. Delamination in helical milling studies of CFRP Paper Rahim et al.

Measurement system Toolmaker's microscope Three-dimensional microscope

Delamination factor model (Fd)

Fd  Dmáx D0

Sultanaa et al.

Optical Microscope

Fd  Dmáx D0 Fd  Dmáx D0

Ishida et al.

Digital microscope

Fd  Ad A0

Wang et al.

Parameters investigated

Position of measurement

Delamination factor

cutting conditions; tool design

borehole exit

1 - 1.6

top layers

1.05 - 1.12

borehole entry and exit

1.04 - 1.4

borehole entry

0.04 - 0.12

progression with tool wear evolution progression with tool wear evolution conventional versus ultrasonic vibration helical milling; coolant conditions

Dmáx = maximum diameter of the damaged area; D0 = nominal diameter; Ad = damaged area; A0 = nominal borehole diameter

Compressive residual stress, which is resultant of mechanical effect, was reported in borehole surface of titanium alloy obtained by helical milling, in contrast with tensile residual stress, resultant of temperature effect, achieved in borehole surface generated by the drilling process. It was also verified the absence of white layer and deformation layer, proving that the mechanical deformation dominates in the helical milling process and that helical milling process can effectively extend the fatigue life of the workpiece [31].

In the helical milling of AISI D2 hardened steel, white layer emerged with the progression of the flank wear. With respect to tool wear evolution, high cutting temperatures were developed due to distortion of the grain boundaries in the direction of feed and deformation of carbide grains at high cutting speeds, and the poor deformability of the carbide grains generated microcracks, further deteriorating the generated surface [49]. Higher hole subsurface microhardness with no plastically deformed layer or white layer has been observed in holes produced by helical milling. In contrast, a slightly softened region was always present on the drilled surface. Tensile residual stress remains in the hole produced by the drilling process, in contrast with a compressive residual stress in the hole produced by helical milling [31]. Boreholes produced in aircraft fuselage may present regions of concentrate stress where fatigue crack can initiate and propagate [86,87]. Fatigue tests were carried out in boreholes obtained by drilling and helical milling under dry and wet conditions in the Ti-6AL-4V alloy. It was demonstrated that helical milling increased the fatigue of boreholes life when compared to drilled boreholes. The fatigue life results were correlated with roughness results of boreholes, since high roughness may contribute to decrease the fatigue life [86]. Similar results on fatigue life were achieved in Ti-6Al-4V and Al 2024-T3 [87], endorsing the superiority of helical milling to obtain boreholes with higher fatigue life when compared to drilled boreholes. Severe subsurface plastic deformation was observed in drilled samples of Al alloy, followed by weakened mechanical properties of the recast layer due to thermal softening, such as lower Young modulus and microhardness. In the case of helical milling for the Ti alloy, wet cut achieved higher fatigue life than the dry condition [87]. The MQL lubrication condition was not evaluated and the cutting conditions of helical milling were not evaluated with regard to subsurface modifications. 8. HELICAL MILLING APPLICATIONS AND SCREENING Helical milling and orbital drilling presents a broad field of application in aircraft/aerospace industry [7,34,40,62,77], in automotive industry [31,61,66,68], in naval industry [28,36,62], in difficult-to-cut materials [2,5,6,22,57] in die/tool steels [3,39,72,73] and other fields for obtaining boreholes with high accuracy. In these fields, the main materials which presents demands of helical milling or orbital drilling for obtaining boreholes are carbon fiber reinforced plastics – CFRP [4,6,22,25,32,40,59,66,71,77,82,88], titanium alloys [19,20,31,53,89], CFRP-titanium compounds [5,9,90], aluminium alloys [12,28,33,35,83,88,91], Al/CFRP/Ti composites [67], 2Al2-T4 alloy [56], AISI D2 steel [3], Die steel Cr12 [73], A3 steel, 20CrMnTi hardened steel and Cr12MoV hardened steel [39], AISI 1045 steel [50], quartz fiber reinforced ceramic [65], electrical steelstacks [85], quartz fiber reinforced ceramic matrix composites [65] and others. Different machines, tools, and lubrication conditions can be applied to obtain boreholes through helical milling or orbital drilling processes, according to the work material properties. Table 7 summarizes these aspects in the helical milling of CFRP. CFRP are highly attractive for use in the aircraft industry due to their high strength-to-weight ratio [9]. Most part of the helical milling experimental works in CFRP were carried out in machining centres [4,22,25,66,78,82], two works were carried out in orbital drilling units [59,88] and one with a hexaglide machine tool [40]. In relation to lubrication conditions, most part of the summarized works of helical milling in CFRP was done in dry condition. To assist chip removal, suction unit [40] and compressed air [66] were applied. An orbital drilling tool with cooling hole was developed [59]. The cooling hole of the tool is located in the tooth back to

prevent blocking caused by the chips around the hole and make sure that the cooling gas will cool rake face of the end cutting edge where tool wear mainly occurs. In helical milling of CFRP, the majority of the works were realized using solid end mills. It is important to notice that in the screening of the helical milling in CFRP the laminate structure was not detailed, then it is important to reference to the original papers to understand this characteristic.

Table 7. Machine, tool and lubrication in helical milling of CFRP Paper Voss et al. [40] Liu et al. [4] Rahim et al. [82] Sakamoto and Iwasa [66] Chen et al. [59] Chen et al. [88] Qin et al. [22] Sadek et al. [25] Sultana et al. [78]

Machine tool

Tool material/type

Lubrication condition

Parallel kinematic hexaglide machine tool DMC75V linear 5-axis high speed machining centre Sodick 430L vertical machining centre Vertical machining centre SV400, V56 Robot automatic orbital drilling KUKA 360-1 with end-effector Orbital drilling unit designed by Zhejiang University fixed to KUKA 360 robot DMC75V linear 5-axis high speed machining centre Makino A88e 5-axis machining centre 5-axis Makino A88e machining centre with IBAG high speed spindle

Cemented carbide with nano-crystalline diamond coating/end mill

Suction unit

Solid cemented carbide/end mill

Dry

Cemented carbide with grade K30/flat end mill and radius end mill

Dry

Cemented carbide/ball nose end mill

Dry, Compressed air

-/Orbital drilling tool with cooling hole

Gas

Cemented carbide/specialized tool and end mill

Dry

TiAlN-coated tungsten carbide cutter

Dry

-/end mill

Dry

Superabrasive diamond tool/ball end mill

-

- not informed

Table 8 presents the machines, tools and lubrication conditions in some applications of helical milling in Ti alloys. Titanium alloys are normally placed inside the aluminium fuselage to prevent the crack growth which could end in catastrophic failure as well as in motor parts [89]. In these works, the Ti alloy was the Ti6Al-4V. In only one experimental work an orbital drilling unit was used [53]. Tools with ultra-fine grain carbide-coated inserts were specifically designed for helical milling tests [31]. The lubrication conditions applied in the helical milling of Ti alloys include dry cutting, where a competing process between work hardening and thermal softening takes place and affects the fundamental behaviour of the workpiece material [31]. In the helical milling of Ti-6Al-4V with air-cooling, it was assumed that temperature change has minimum impact in the workpiece. Then, in order to avoid the thermal damage, the air coolant may be applied [92]. Some cases of helical milling of Al alloys are presented in Table 9. For instance, the aluminium 7075T6 alloy is used in the fabrication of several parts of commercial aircraft, including fuselage sections [12]. In relation to the tool geometry, different end mill geometries have been tested [83] and developed [88,91] to perform helical milling in Al alloys. Some experimental works applied the dry condition in the helical milling of Al alloys [33,83,88]. However, the aluminium presents high ductility and easy adherence to the cutting tool, then a wet coolant system is usually employed. Consequently, the wet coolant, air blow, and MQL strategies were compared, showing that the shape error is smaller, the burr formation is avoided and the temperature is lower with MQL application. The helical milling of Al 7075-T6 with MQL presented machining accuracy comparable with the helical milling with wet coolant [35].

Table 8. Machine, tool, alloy and lubrication in helical milling of Ti alloys Paper

Machine tool

Zhao et al. [31]

DMC75V linear 5-axis high speed machining centre

Urbicain et al. [89]

-

Olvera et al. [19]

Ibarmia ZV 5-axis machining centre

Rey et al. [53]

Ji et al. [20] Liu et al. [92]

Specific drill bench equipped with an orbital spindle, property of AIRBUS DMC75V linear 5-axis high speed machining centre DMC75V linear 5-axis high speed machining centre

Ti alloy/ Hardness

Lubrication condition

Ti-6Al-4V/330 HV

Dry

Ti-6Al-4V/36 HRC

-

AlTiN-coated carbide/ball end mill

Ti-6Al-4V/36 HRC

External flood coolant Hocut B-750 semi-synthetic soluble fluid

-

Ti-6Al-4V/-

-

TiAlN-coated carbide/end mills

Ti-6Al-4V/-

Air-cooling

-

Ti-6Al-4V/-

Air-cooling

Tool material/type TiAlN-coated tools with ultra-fine grain carbidecoated/inserts TiAlN-coated cemented carbide/ ball end mill

Table 9. Machine, tool, alloy and lubrication in helical milling of Al alloys Paper

Machine tool

Tool material/type

Al alloy

Lubrication condition

Shan et al. [33]

Lathe CA6140

Solid cemented carbide/end mill

Al 6061

Dry

Sasahara et al. [35]

Machining centre

-

Al 5052

Wet; MQL; Air blow

Chen et al. [88]

Orbital drilling unit designed by Zhejiang University fixed to KUKA 360 robot

Al 7050

Dry

Brinksmeier and Fangmann [83]

Schmid Orbital Drilling Unit (ODU)

Cemented carbide/specialized tool and end mill Cemented carbide without coating/end mill - five different tool geometry

Primed clad aluminium Al 2024-T351

Dry

Zhongqun and Qiang [28]

Vertical CNC machining centre FIDIA K197

Al 7075-T6

High-pressure air blast delivered through a nozzle

Zhang et al. [91]

Five-axis NC machine centre DMU70evo of DMG Corp

Al 7075-T6

-

Pereira et al. [11]

Romi Discovery 560

Al 7075-T6

Wet

Solid carbide/end mill Carbide grade K10/customized end mill Solid carbide/end mill

CFRP-Ti stacks present some advantages over stack-ups made of aluminium and CFRP, such as similar thermal expansion, reduced galvanic corrosion issue, and higher specific strength. However, CFRP-Ti stacks are difficult-to-cut due to high tool wear, fibre delamination, different diameter tolerances caused by the different material properties, reducing the borehole quality [9]. Some experimental apparatus of helical milling of CFRPTi stacks are presented in Table 10. For instance, a special cutting tool was designed to achieve higher machining quality in the orbital drilling of CFRP/Ti stacks with cooling holes to accelerate chip removal and cooling [90]. The helical milling of other materials was also summarized in terms of machine tools, tools, and lubrication of some different applications in Table 11. Considering the difficulty of hard drilling, the helical milling was applied in hardened AISI D2 by using indexable ball nosed inserts and flat nosed solid carbide tools [3]. Air blow was employed to assist chip transport [3].

Table 10. Machine, tool, alloy and lubrication in helical milling of CFRP-Ti stacks Paper Wang et al. [5] Denkena et al. [9] Zhou et al. [90]

Machine tool DMC75V linear 5axis high speed machining centre Heller MC16 4axis machine tool

Tool material/type

CFRP/Ti alloy

Lubrication condition

TiAlN-coated carbide tool

CFRP/Ti6Al4V stacks

Dry

TiAlN-coated solid carbide/end mill TiAlN-coated solid hard Orbital drilling unit alloy (YG8) tool designed fixed to by the authors for CFRP/Ti KUKA360-1 robot orbital drilling

CFRP/Ti6Al4V stacks CFRP/Ti alloy (TC4-DT) stacks

Dry with vacuum dust chip removal system Blowing air through tool cooling holes for Ti chips removal and vacuum dust removal system for CFRP chips

The correct selection of machining parameters to ensure the dimension accuracy, chatter avoidance, tool life prolongation, and the productivity [8]. To apply the helical milling process in different materials is a difficult task due to the selection of cutting conditions. Due to the diversity of the machining operations and work materials, the manufacturers cannot suggest specific parameters for specific process such as helical milling. Consequently, the engineers need to test experimentally to achieve appropriate levels of each cutting parameter, respecting the constraints of the tool and machine. For a first suggestion, a screening of the cutting conditions of the experimental applications literature of helical milling may attend this task.

Table 11. Machine, tool, alloy and lubrication in helical milling of other materials Tool material/type

Material

Lubrication condition

Solid carbide/end mill

2Al2-T4

-

-/Indexable tool

Solid bar AISI 1045 steel

5% of semisynthetic oil in water

Customized desktop CNC milling centre

Solid micrograin carbide/end mill

M-19 electric steel stacked with layers of epoxy resin

-

Iyer et al. [3]

Makino MC56-5XA horizontal machining centre

TiCN/TiN and TiAlNcoated/indexable insert ball nose end mills. TiAlN-coated - P05 and P20 grades/flat nosed solid carbide end mill

AISI D2 plates with 60 HRC

Dry with an air blow at a pressure of 8 bar

Wang and Qin [74]

Five-axis machining centre MAZAK

-

Cr12 plate with 55 HRC

-

Paper Shen et al. [56] Costa et al. [50]

Liles and Mayor [85]

Machine tool VMC0850B machining centre Machining centre Romi Discovery 4022

About the different applications of helical milling in the literature, some difficulties were found to standardize the helical milling cutting data. Unfortunately, there are several works without complete cutting conditions, making it difficult to reproduce these experimental studies. Another difficulty is due to the lack of standardization of helical milling parameters nomenclature. Although it is clear that the several works available in the literature are complementary, some authors use to set its own nomenclature for parameters already described in previous works, making it difficult to understand the helical milling kinematics and delaying the development of the research in this field. To help engineers decide about the helical milling use in different materials, the Tables 12-15 present cutting conditions applied in different experimental works of the helical milling process. This screening was done considering only works which provided full information about cutting parameters. Besides, the cutting data were standardized and, the helical milling conditions were calculated considering the decomposition proposed

by Denkena et al. [9]. Then, the feeds per tooth fza and fzt and the cutting velocity vc are provided to present different works in the same perspective. In some papers, the tangential feed per tooth fzt was considered in TCP, i.e., in the helical diameter Dh. However, in circular trajectories the chip load will vary according to the diameter of the point of contact, fzt was calculated with consideration to the borehole diameter, as proposed by Denkena et al. [9], assuring the highest feed per tooth in the circular tool path, guaranteeing that the chip load does not exceed the recommended limit for the tool. With these basic helical milling parameters, the helical milling conditions for CNC programming, such as vf, n and ap* can be easily obtained considering the Equations 1-8. As the cutting conditions are generally chosen taking into consideration the work and tool materials, the Tables 12-15 need to be checked together with the Tables 7-11 and, eventually, the original papers. Furthermore, the tool constraints need to be respected, i.e., it is important to respect the chip load, the cutting velocity and the maximum depth of cut for each specific tool, according to the tool manufacturers. The different papers summarized in this screening present dissimilar objectives in terms of helical milling performance. For instance, there are papers dedicated only to cutting force studies [2,51,74], works related to surface roughness evaluation [28,39,73], experimental works dealing with geometrical and dimensional error assessment [72,90] and works which considered several helical milling responses [3,61]. Consequently, for a better use of this screening, it is recommended to check the purpose of the original investigations and examine if it matches your own objectives for the correct application of cutting parameters of the helical milling process. Table 12 presents the helical milling cutting conditions applied in experimental work in CFRP. In these works, the variation of borehole diameter was from Db = 6.4 to Db = 15 mm, with tool diameter in the range of Dt = 5.0 to Dt = 10.0 mm. The axial feed per tooth varied from fza = 0.064 to fza = 24.3 µm, while the tangential feed per tooth was in the range of fzt = 10 to fzt = 1050 µm. The cutting velocity was from vc = 57 m/min to vc = 440 m/min. This high variation can be explained considering the differences in the experimental setup, CFRP construction, tool material and geometry, work material, and investigations purposes of each paper. Cutting conditions for helical milling in Ti alloys are summarized in Table 13. The borehole diameter variation was from Db = 9 to Db = 11.1 mm with tool diameter variation from Dt = 6.0 to Dt = 9.0 mm. The axial feed per tooth was from fza = 0.12 to fza = 50 µm while the tangential feed per tooth was from fzt = 40 to fzt = 270 µm. The cutting velocity was from from vc = 30 m/min to vc = 100 m/min. In these works, the variation in the diameter of the boreholes and tool was lower than in CFRP cases, implying in a lower variation of the applied cutting conditions. For helical milling of Al alloys, the cutting conditions of some experimental cases are in Table 14. The borehole diameter variation was from Db = 10.2 to Db = 16 mm with tool diameter variation from Dt = 8.0 to Dt = 10.0 mm. The cutting parameters variation was fza = 0.2 to fza = 20 µm, fzt = 50 to fzt = 204 µm and vc = 38 m/min to vc = 785 m/min. It is important to underline that the aluminium alloy varied from one work to another according to Table 9. Finally, helical milling parameters of experimental cases with other work materials are summarized in Table 15. The applications in CFRP-Ti stacks and in D2 hardened steel can be highlighted due to the importance of these materials in the aircraft, and molds and dies manufacturing industries, respectively.

Table 12. Helical milling cutting conditions in CFRP Paper Voss et al. [40,93] Liu et al. [4] Sakamoto and Iwasa [66]

Borehole geometry Dt Dh (e) [mm] [mm] 5.0 1.4 (0.7) 6.0 4.0 (2.0)

Db [mm] 6.4 10.0 12.0

6.0

6.0 (6.0)

Qin et al. [22]

12.0 - 15.0 10.0

10.0 6.0

2.0 - 5.0 (1.0 - 2.5) 4.0 (2.0)

Sadek et al. [25]

9.5

6.35

3.15 (1.575)

Chen et al. [88]

Helical milling parameters z fza fzt vc [µm/tooth] [µm/tooth] [m/min] 3 3.4 80 160 4 0.064 - 0.382 10 - 60 57 2

3.99 - 11.98

151 - 452

4 0.179 - 1.875 4 0.064 - 0.318

34 - 177 20 - 40

4

56 - 75

2.5 - 5.6

100 314 440 75 - 151 120 319

Table 13. Helical milling cutting conditions in Ti alloys Paper

Db [mm] 10.0 9.0 11.1 10.0

Zhao et al. [31] Olvera et al. [19] Rey et al. [53] Liu et al. [92]

Borehole geometry Dt Dh (e) [mm] [mm] 6.0 4.0 (2.0) 7.0 2.0 (1.0) 9.0 2.1 (1.05) 6.0 4.0 (2.0)

z 4 2 3 4

Helical milling parameters fza fzt vc [µm/tooth] [µm/tooth] [m/min] 0.64 100 66 5.16 65 100 5 211 30 0.64 100 62 - 74

Table 14. Helical milling cutting conditions in Al alloys Paper Shan et al. [33] Sasahara et al. [35] Chen et al. [88] Zhongqun and Qiang [28] Pereira et al. [11]

Borehole geometry Db Dt [mm] [mm] 10.2 - 12.2 8.0 - 10.0 15.0 10.0 15.0 10.0

Dh (e) [mm] 2.2 (1.1) 5.0 (2.5) 5.0 (2.5)

z 2 2 4

Helical milling parameters fza fzt vc [µm/tooth] [µm/tooth] [m/min] 2.7 - 5.3 171 38 0.635 30 785 1.5 0.35 94

16.0

10.0

6.0 (3.0)

2

20

336 - 837

38

15.0

10.0

5.0 (2.5)

4

0.5 - 14.5

15 - 115

10 - 90

Table 15. Helical milling cutting conditions in other materials Borehole geometry Db Dt Dh (e) [mm] [mm] [mm]

z -

Helical milling parameters fza fzt vc [µm/tooth] [µm/tooth] [m/min] 2.0 - 12.0

Paper

Workpiece material

Denkena et al. [9]

CFRP-Ti

10.0

8.0

2.0 (1.0)

3

Iyer et al. [3]

AISI D2 (60 HRC)

16.0

12.0

4.0 (2.0)

Wang and Qin [74]

Cr12 (55HRC)

10.0

6.0

4.0 (2.0)

40 - 120

40

2 1.3* / 0.43**

100* / 33**

30 - 47* / 66**

4

106 - 148

70 - 120

0.67 - 0.86

*indexable ball nose end mill; **solid carbide end mill

The cutting parameters presented for different helical milling cases are not optimized conditions, although these levels were applied with success to achieve each investigation objectives. The best levels of helical milling cutting conditions for each application may be achieved through modelling and optimisation techniques. There is a lack of investigations about helical milling optimisation, and the few ones were described

in section 5. The optimisation of helical milling process could achieve improved borehole quality in terms of dimensional, geometrical and microgeometrical errors; cutting forces and energy consumption reduction; and high the productivity. These responses could be considered individually or in multi-objective scenarios. 9. CONCLUSONS AND FUTURE OPPORTUNITIES The present paper summarizes the state of the art of helical milling process, with regard to advantages, kinematics, cutting force, temperature, tool life and wear, statistical and soft computing in helical milling, borehole quality, and helical milling applications. As a first review paper about helical milling, this paper can be referenced to point out future work opportunities about helical milling process. Several advantages of this process were presented with regard to drilling since both are hole-making machining process. Advantages with regard to quality, chip removal process, cutting forces, lubrication, and inventory economy were described. Despite low productivity with regard to drilling, with helical milling finished boreholes may be obtained with just one operation, avoiding setups and subsequent finishing operations, such as reaming process. Boreholes obtained by drilling need a secondary process to improve the quality in terms of dimensional, geometrical, and microgeometrical error. Then, more studies with focus on the comparison between helical milling and reaming to define the best process in terms of borehole quality are necessary. Helical milling represents economy in inventory, setup, and number of operations and a hole-making strategy with conventional drilling and reaming may, consequently, represent higher processing costs. When comparing these two strategies, if it is concluded that boreholes obtained by helical milling in only one operation may present lower levels of quality when compared to boreholes finished by reaming, it is also important to test the helical milling in two operations with the same tool, the first for borehole generating and the second for finishing to achieve better borehole quality. The kinematics was presented in different perspectives and with the proposal of nomenclature standardization. This will help engineers to understand the process, to reproduce the present literature and conduct future investigations based on the current state of the art. Besides, the undeformed chip parameters were reported and the cutting volumes of the peripheral and frontal cut were summarized in function of the diameters of borehole and of the helix. However, the influence of the machine-tools acceleration and deceleration in the kinematics of helical milling should be taken into account, mainly in high-speed cutting. During helical milling, when the linear axes change the movement direction in the interpolation, the acceleration and deceleration may lead to geometrical and dimensional errors. Helical milling screening was realized considering different materials. This summary should help researchers and engineers to apply helical milling to machine boreholes in different materials. Most of the works was performed in CFRP, Ti-6Al-4V, CFRP-Ti stacks, and Al alloys, especially due to aerospace industry necessities. There are several materials which need hole-making operations and should be tested in helical milling investigations. For instance, nickel-based superalloys, also important in the aerospace industry, require micro-holes in the manufacture of cast turbine blades [94]. Furthermore, there are lots of metallic and composite materials in other manufacturing fields which requires hole-making operations and the helical milling process may be better explored.

Boreholes obtained in molds and dies are produced by traditional drilling processes and a study of helical milling with focus on molds and dies will provide important information about helical milling in hardened steel. Moreover, it is important to consider others aspects when talking about hole-making in molds and dies machining. To avoid setups, a ball nose end mill, generally used in molds and dies machining, may be used for hole-making by helical milling. Then, considering that some cavities require high tool overhang length, to avoid collisions, the effect of tool overhang length and deflection of the tool in dimensional and geometrical error in boreholes obtained through helical milling should also be studied. The characterization of boreholes obtained by helical milling should be improved in investigations, taking into account mechanical and metallurgical properties. Comparisons with drilling were realised. However white layer, plastic deformation, grains orientation, microhardness, cracks and others aspects should be addressed with regard to the effect of cutting conditions of the helical milling process. Micro helical milling was not investigated and there are a lot of requirements which may be studied, since cutting forces, borehole quality, tool wear, tool life, mechanical, and metallurgical characterization. Wear and tool life in helical milling studies were summarized, with respect to work material, tool material, tool coating, wear monitoring technique, peripheral and frontal cutting edges wear, different types of wear and wear mechanisms, tool life criteria and number of boreholes obtained. As these studies addressed to wear in CFRP, Ti-6Al-4V, and AISI D2 steel, there are several future wear investigation opportunities in helical milling of other difficult-to-cut materials, such as other hardened steels. It is also important to better understand the wear mechanism in the helical milling and to define mathematical models to predict wear in function of helical milling parameters. Cutting force and temperature studies were also discussed. Once these outcomes are generally correlated with tool wear, dimensional, geometrical and microgeometrical deviations, future works may attempt on cutting force and temperature minimization. Cutting force increase may accelerate the wear and also cause dimensional and geometrical deviation. High temperature levels may reduce tool life and weaken mechanical properties of the subsurface of boreholes obtained by helical milling. Quality in terms of dimensional error, roundness, roughness, delamination, burr and other aspects was summarized in the helical milling of different applications. These results endorse the capability of helical milling of manufacture finished boreholes. The correlation of radial cutting force with dimensional error in helical milling was confirmed. Besides, it was demonstrated that the flank wear in the peripheral cutting edges also caused dimensional deviation in the helical milling of CFRP. The possibility of tool path adjustment for dimensional deviation was mentioned, however, was not experimentally studied. In most of the papers, experimental results were not used to obtain mathematical and statistical models for these quality responses, in terms of helical milling parameters. Then, upcoming investigations should also address to modelling and optimisation with subsequent experimental confirmation, to achieve the best quality levels of helical milling. ACKNOWLEDGEMENTS The authors gratefully acknowledge CNPq, CAPES, and FAPEMIG for supporting this research. The first author acknowledges CAPES for the PDSE grant, process number 88881.133263/2016-01.

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Highlights

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A review of helical milling process is presented. Helical milling advantages, kinematics, undeformed chip and cutting volumes are discussed. A screening of helical milling experimental studies is presented. Helical milling results about wear, hole quality, cutting force and temperature are presented. Recent results and future trends about helical milling are presented.